Phylogenetic comparative methods (PCMs) use information on the evolutionary relationships of organisms (phylogenetic trees) to compare species (Harvey and Pagel, 1991). The most common applications are to test for correlated evolutionary changes in two or more traits, or to determine whether a trait contains a phylogenetic signal (the tendency for related species to resemble each other). However, several methods are available to relate particular phenotypic traits to variation in rates of speciation and/or extinction, including attempts to identify evolutionary key innovations. Although most studies that employ PCMs focus on extant organisms, the methods can also be applied to extinct taxa and can incorporate information from the fossil record.

What distinguishes PCMs from most traditional approaches in systematics and phylogenetics is that they typically do not attempt to infer the phylogenetic relationships of the species under study. Rather, they use an independent estimate of the phylogenetic tree (topology plus branch lengths) that is derived from a separate phylogenetic analysis, such as comparative DNA sequences that have been analyzed by maximum parsimony or maximum likelihood methods. PCMs are consumers of phylogenetic trees, not primary producers of them. Accordingly, the list of phylogenetics software shows little overlap with the programs for PCMs (see below).

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PCMs can be used to analyze the origin and maintenance of biodiversity. Biodiversity is most commonly discussed in terms of the number of species, but it can also be phrased in terms of the amount of phenotypic (e.g., physiological, morphological) space that a given set of species occupies (see also Cambrian explosion).

The algorithm involves computing values at internal nodes as an intermediate step, but they are generally not used for inferences by themselves. An exception occurs for the basal (root) node, which can be interpreted as an estimate of the ancestral value for the entire tree (assuming that no directional evolutionary trends [e.g., Cope's rule] have occurred) or as a phylogenetically weighted estimate of the mean for the entire set of tip species (terminal taxa). The value at the root is equivalent to that obtained from the "squared-change parsimony" algorithm and is also the maximum likelihood estimate under Brownian motion. The independent contrasts algebra can also be used to compute a standard error or confidence interval.

Martins and Garland (1991) proposed that one way to account for phylogenetic relations when conducting statistical analyses was to use computer simulations to create many data sets that are consistent with the null hypothesis under test (e.g., no correlation between two traits, no difference between two ecologically defined groups of species) but that mimic evolution along the relevant phylogenetic tree. If such data sets (typically 1,000 or more) are analyzed with the same statistical procedure that is used to analyze a real data set, then results for the simulated data sets can be used to create phylogenetically correct (or "PC" [Garland et al., 1993]) null distributions of the test statistic (e.g., a correlation coefficient, t, F). Such simulation approaches can also be combined with methods like phylogenetically independent contrasts (see above).

In addition to having a long and extremely productive history in biology, comparative methods are often controversial. Rather than attempt to recount the various controversies, many of which are ongoing, quotes (in chronological order) are illustrative.

"Ought we, for instance, to begin by discussing each separate species - man, lion, ox, and the like - taking each kind in hand independently of the rest, or ought we rather to deal first with the attributes which they have in common in virtue of some common element of their nature, and proceed from this as a basis for the consideration of the separately?" (Aristotle, De partibus animalium; quoted in Harvey and Pagel, 1991, p. 35)

"In parallel with laboratory experimental methods, the comparative method increases in value with its sample size, i.e., the number of species being compared. When species are few and their phylogenetic relationships are clouded in the distant past of unfossilized ancestors, the comparative method reaps fewer conclusions of trust." (Hailman, 1976, p. 20)

"... there is no easy way, except by comparison, to test most questions about the long-term history of life, or to generate predictions from evolutionary considerations." (Alexander, 1979, p. 13)

"... we must learn to treat comparative data with the same respect as we would treat experimental results ..." (Maynard Smith and Holliday, 1979, p. vii)

"In the past, however, cooperation between systematists and other comparative biologists has been sporadic at best. Most experimental biologists have ignored taxonomy and systematics, some even to the extent of not bothering to provide their animals with proper identifications or scientific names." (Atz et al., 1980, p. 7)

"... biology will never secure fossils of all species in the past because fossilization is such a rare process, requiring just the right physical and chemical conditions. Therefore, in order to trace probable phylogenetic lineages one must reason from the evidence at hand: the characteristics of contemporary animals themselves which are the end-points of phylogeny (evolutionary history)." (Hailman, 1981, p. 93)

"... the comparative method of 1950 was indistinguishable from the comparative method of 350 BC ..." (Ridley, 1983, p. 6)

"Focusing only on highly adapted species may, of course, bring valuable information on extreme situations but might also obscure the basic mechanisms." (Bankir and Rouffignac, 1985, p. R663)

"Most of what we know is based upon comparison. When asked to describe a food not previously tasted or a new kind of music, one often responds that the taste is "like" some other food, or that the sensation "differs" in a particular way from something that is familiar. Indeed, comparison and the similarities and differences it discloses is ingrained in our approach to description of objects, events and processes. Hence the questions "what can we compare?" and its ancillary "how shall we compare?" prove to be the key to any study of natural phenomena." (Gans, 1985, p. 291)

"Some reviewers of this paper felt that the message was "rather nihilistic," and suggested that it would be much improved if I could present a simple and robust method that obviated the need to have an accurate knowledge of phylogeny. I entirely sympathize, but do not have a method that solves the problem. The best we can do is perhaps to use pairs of close relatives as suggested above, although this discards at least half of the data. Comparative biologists may understandably feel frustrated upon being told that they need to know the phylogenies of their groups in great detail, when this is not something they had much interest in knowing. Nevertheless, efforts to cope with the effects of the phylogeny will have to be made. Phylogenies are fundamental to comparative biology; there is no doing it without taking them into account." (Felsenstein, 1985, p. 14)

"Comparative biologists tend to suspect comparisons of distantly related species; they hope to base their comparisons on recent evolutionary events that have not been overlaid by much subsequent change." (Felsenstein, 1988, p. 465)

"Yet, one of the most embarrassing things that could be done to a group of respected biologists would be to ask them to spend a few minutes to write down what is meant by the comparative method, and what are the basic goals and principles of biological comparison." (Bock, 1989, p. 18 [cited in Starck, 1998, p. 110])

"As welcome as it is to see the lung successfully employed as a systematic index, it is, on the other hand, unfortunate. Thereby, lungs lose their innocence in the sense that phylogenetic trees and cladograms can no longer be used to help resolve the sequence in the development of lung structure without the danger of circular argumentation." (Perry, 1989, p. 200)

"To be maximally informative, such studies should be undertaken on closely related groups of organisms, so that factors extraneous to the comparison can be minimized ..." (Bennett and Huey, 1990, p. 272)

"The comparative approach is not new. Indeed, it was Darwin's favoured technique. ... In short, comparative studies have taught us most of what we know about adaptation." (Harvey and Pagel, 1991, p. v)

"In short, all useful comparative methods are based on explicit models of evolutionary change." (Harvey and Pagel, 1991, p. v)

"The usual symptom of non-independence is that closely related species tend to be more alike than more distantly related species." (Harvey and Pagel, 1991, p. 81)

"... to use species as independent data points in a comparative analysis requires that one ignores phylogenetic relationships." (Harvey and Pagel, 1991, p. 122)

"Because life-history traits are likely to be correlated with a species' phylogenetic history, unequivocal evidence for adaptation to local environmental conditions may be recognized only after the variation in a trait attributable to phylogeny is removed." (Miles and Dunham, 1992, p. 848)

"Broad-scale comparative evidence from across a large number of taxa may often help set limits to the applicability of hypotheses that have been generated from a particular phenomenon in a particular species." (Moller and Birkhead, 1992, p. 650)

"However, in general, the evolutionary process involves descent with modification, and in the absence of modification, one must conclude that similarities among closely related taxa reflect shared ancestry. Phylogeny, then, is an important explanatory principle for understanding shared characteristics and should be the null hypothesis in all tests of similarity or differentiation among taxa." (Di Fiore and Rendall, 1994, p. 9945)

"In addition to his theory of natural selection, the comparative method is what made Darwin great. If you don't believe this claim, look at any of his major works. They are packed full with interspecific comparisons based on detailed studies and anecdotal observations." (Hauser, 1996, p. 10)

"Population comparisons can provide particularly powerful means of evaluating adaptive hypotheses for two reasons. The first is that there tend to be fewer differences between populations than between species. Consequently, there are fewer covarying traits to confound analyses ... Second, divergent populations are often relatively young and may be more likely to reside in the habitats in which their derived character states evolved than is the case for divergent species with potentially longer intervening histories ..." (Foster and Cameron, 1996, p. 140).

"Naive, prephylogenetic comparative tests should be kept at the other end of a barge pole." (Ridley and Grafen, 1996, p. 87)

"It is the study of the bizarre, the outliers, the freaks, that gives us some of our clearest insights into the hows and whys of evolution." (Torr, 1998, p. 52)

Ackerly, D. D. 1999. Comparative plant ecology and the role of phylogenetic information. Pages 391-413 in M. C. Press, J. D. Scholes, and M. G. Braker, eds. Physiological plant ecology. The 39th symposium of the British Ecological Society held at the University of York 7-9 September 1998. Blackwell Science, Oxford, U.K.